This invention relates generally to the art of interferometry and more particularly to an improved interferometric method and apparatus that is characterized by a wide linear range.
The phase difference between two portions of a light beam can be measured interferometrically. In most systems, wherein it is desired to produce an electrical signal corresponding thereto for control, recording, or display purposes, the optical beams are intensity modulated at a predetermined frequency and photo diode detected to provide two sine wave modulated electrical signals having an electrical phase difference that is a function of the optical phase difference. These electrical signals are then processed with a phase detector to obtain the optical phase difference. A problem arises when the phase difference is greater than .+-. .pi.. Since sine waves repeat themselves, it is difficult to measure, with the foregoing systems, optical phase differences that give rise to multiples of .pi., primarily because of the modulo 2.pi. ambiguity that exists.
In the case of analyzing interferograms, for example to determine the surface quality or optical figure of a mirror, it is desirable, indeed necessary as a practical matter, to be able to detect surface anomalies from point to point that give rise to multiple interference fringes as well as fractions of a fringe. This, of course, requires a wide linear range. Extensions of linear range in interferometric apparatus have been accomplished heretofore by using dividers and counters to, in effect, digitally count fringes or lines from an assigned starting point. While such systems have been beneficial in certain laboratory and industrial applications, they suffer the disadvantages of being inherently electrically "noisy" and of being limited in speed of operation. These disadvantages become important, for example, in a system for detecting and control of wavefront distortion in a high energy laser beam. The practicality of such a system requires interferometric determination of the phase distortions of the beam wavefront and generation of suitable electrical control signals for actuating wavefront correction means, such as a deformable mirror, so as to remove the distortions.